The Enhancement of Phase‐Matching Capability by Recombining π‐Conjugated Group in Nonlinear Optical Sulfates

ABSTRACT Short‐wavelength ultraviolet (UV) nonlinear optical (NLO) crystals are critical components in all‐solid‐state UV lasers, as they could extend the tunable range of lasers. NLO sulfates always exhibit a large bandgap and a moderate second harmonic generation response, which should be a rich source of NLO materials. However, due to the high symmetry of the [SO ₄ ] ²⁻ tetrahedron, most sulfates present insufficient phase‐matching capability. In terms of this, we proposed the π‐conjugated [C(NH ₂ ) ₃ ] ⁺ modulating strategy to improve the birefringence of sulfates, and obtained two NLO crystals, [C(NH ₂ ) ₃ ] ₄ Cl ₂ SO ₄ (GCS) and [C(NH ₂ ) ₃ ] ₄ Br ₂ SO ₄ (GBS). They both exhibit a moderate birefringence (0.050 for GCS and 0.052 for GBS at 546 nm), while retaining short UV cutoff edges. Besides, GBS presents a strong second‐order NLO response (1.2 × KH ₂ PO ₄ ), resulting from the synergy of almost uniformly arranged π‐conjugated planar [C(NH ₂ ) ₃ ] ⁺ cations, [SO ₄ ] ²⁻ groups, and halogen anions. These results confirm the validity of the strategy. Therefore, this work provides a novel perspective in improving the sulfate's phase‐matching capability, and further expands the research field of UV NLO crystals.